It is common in scanning optical systems used for reading and recording information to employ a rotating polygonal mirror as a light reflector. The polygonal mirror typically comprises a plurality of precisely angled reflecting surfaces (facets) assembled about an axis in an equilateral polygon configuration. Rotation of the polygonal mirror about its axis will cause a light beam incident thereon to be sequentially reflected by each of the facets and repetitively swept with each reflection through a predetermined arc. Optics in the form of lenses and reflectors may also be included in the system to direct the swept beam to scan across the surface of an object.
It is also known to reflect the repetitively swept beam back to the rotating polygonal mirror where the beam is again sequentially reflected by each of the facets and repetitively swept through another predetermined arc. Additional optics may then be utilized to receive and focus the swept beam for scanning of the surface of the object. In such a system, the included optics are further employed to shape the swept beams generated by both the first and second polygonal mirror reflections such that the beam swept by the first reflection by the polygon is expanded to track the rotational movement of the polygon facets at the second reflection. U.S. Pat. Nos. 4,624,528; 4,129,355; 4,030,806 and 3,972,583 are illustrative of the use of a double reflecting scanner system and included optics.
Due to the nature of the rotating polygonal mirror, the point on each reflective surface (facet) of the polygonal mirror where reflection of the light beam occurs is longitudinally shifted with respect to the light path of the incident beam as the polygonal mirror rotates and the reflected beam is swept through the predetermined arc. If optics are included in the system for receiving the swept beam reflected by the rotating polygonal mirror, the entrance pupil for the included optics will also be shifted, in a manner corresponding to the point on the reflective surface, as the polygonal mirror rotates. The shifting (or distortion) of the position of the entrance pupil caused by the rotation of the polygonal mirror results in a field curvature at the image plane of the included optics that adversely distorts the swept beam.